I'm not sure if I understand entirely what you're asking, but I think it comes down to "planetary differentiation". It's the same process by which water floats on top of oil because of density. The heavy stuff floats to the bottom and the light stuff floats to the top. That process isn't 100% complete obviously, but that's because near the surface material strength matters a great deal. We still have mountains on Earth, which are clearly not behaving like a liquid. If it was behaving like a liquid, then you're right that the heavier parts would upturn the lighter parts and sink. But Earth still has density variations because the process isn't complete, and tectonic movement keeps rearranging it all.
The Earth is highly plastic ... on geological timescales. The continents move, the Himalaya are rising at the rate of about 5 mm per year, and one theory of the post-Messinian flooding of the Mediterranean basin (https://en.wikipedia.org/wiki/Post-Messinian_flood_of_the_Me...) is that a fragment of one of the colliding plates in the region broke off and sank into the lithosphere.
